Chemistry Reference
In-Depth Information
Fig. 5.4 Mechanism for
water dimer diffusion on
Pd(111) (side and top view).
Step (a) to (b) involves a
nearly free rotation of the
dimer; step (b) to (c) is the
wagging motion of the dimer,
which brings both water
molecules to a similar height
above the surface from where
they can undergo donor-
acceptor tunneling
interchange (c)-(e). From
step (e)to(f), the dimer
restores its equilibrium
geometry having translated
one lattice spacing [compare
(a) and (f)]. Reprinted with
permission from Ref. [
12
].
Copyright 2004, American
Physical Society
interchange process eventually leads to a considerably high diffusion rate of a dimer,
which can explain the experimental findings. In summary, they presented the tun-
neling interchange has a significant impact on the diffusion of a water dimer on
Pd(111) and concluded as follows; (i) For dimers a crossover from a quantum to a
classical diffusion regime is expected at low temperatures. (ii) Diffusion of dimer
should exhibit an isotope effect (a D
2
O dimer should diffuse 10 times more slowly
than an H
2
O dimer in the tunneling regime) (iii) Dimers diffuse faster than monomers
below a specific temperature. Although this finding shed light on the role of quantum
dynamics of H-bond within small water clusters on surfaces, the direct observation of
such process had never reported so far.
